Limnol. Oceanogr., 47(6), 2002, 1774–1789
q 2002, by the American Society of Limnology and Oceanography, Inc.
Routes and rates of larval fish transport from the southeast to the northeast
United States continental shelf
Jonathan A. Hare
NOAA, NOS, NCCOS, Center for Coastal Fisheries and Habitat Research, 101 Pivers Island Road, Beaufort,
North Carolina 28516
James H. Churchill
Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543
Robert K. Cowen
Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, 4600 Rickenbacker Causeway,
Miami, Florida 33149
Thomas J. Berger
Science Applications International Corporation, 615 Oberlin Road, Raleigh, North Carolina 27605
Peter C. Cornillon
Graduate School of Oceanography, University of Rhode Island, South Ferry Road, Narragansett, Rhode Island 02882
Paul Dragos
Battelle Ocean Sciences, 397 Washington Street, Duxbury, Massachusetts 02332
Scott M. Glenn
Institute of Marine and Coastal Sciences, P.O. Box 231, Rutgers University, New Brunswick, New Jersey 08901
John J. Govoni
NOAA, NOS, NCCOS, Center for Coastal Fisheries and Habitat Research, 101 Pivers Island Road, Beaufort,
North Carolina 28516
Thomas N. Lee
Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, 4600 Rickenbacker Causeway,
Miami, Florida 33149
Abstract
Larval fish originating south of Cape Hatteras, North Carolina, are frequently found on the continental shelf north
of Cape Hatteras, even as far north as the Scotian shelf. The Gulf Stream and associated warm-core rings are
hypothesized as the physical mechanisms responsible for the northward transport of larvae. Specifically, larvae
spawned along the southeast U.S. continental shelf are entrained into the Gulf Stream, transported to the northeast,
regularly incorporated in warm-core ring streamers, transported across the slope region, and released along the shelf
edge north of Cape Hatteras. This proposed transport route was evaluated using hydrographic data and drifter tracks
that were collected as part of other studies. Continental shelf water that originated south of Cape Hatteras was found
Acknowledgments
Patricia Tester, Cecily Natunewicz, and David Chapman provided critical reviews of earlier drafts of this manuscript. The South Atlantic
Bight Recruitment Experiment was funded by NOAA Center for Sponsored Coastal Ocean Research, Coastal Ocean Program. The Cape
Hatteras Field Study was funded by the U.S. Department of the Interior, Minerals Management Service contract 14-35-0001-30599 to
Science Applications International Corporation. The Frontal Eddy Dynamics Experiment was supported by the U.S. Department of the
Interior, Minerals Management Service contract 14-12-0001-30349. Partial support for this work was provided by NOAA/CIMAS through
the South Florida Ecosystem Restoration Prediction and Modeling Program, Contract NA67RJ0149. The larval fish work on the northeast
United States continental shelf was supported by the NOAA Office of Sea Grant, U.S. Department of Commerce, under grants NA86AAD-SG045 and NA90AA-D-SG078. Analyses at Woods Hole Oceanographic Institution were supported by the U.S. Department of Energy
through contract DE-FG02-92ER61418. This work was performed while the senior author held a National Research Council Research
Associateship. This is Woods Hole Oceanographic Institution Contribution 10372.
1774
Lagrangian views of larval transport
1775
along the edge of the Gulf Stream north of Cape Hatteras, and drifter tracks demonstrated each segment of the
proposed transport route. A probability model was then developed to estimate transport times for larvae carried
over the proposed transport route using a combination of sea surface temperature images, statistics of drifter transport
speeds, and a larval mortality function. Modeled transport time distributions closely matched observed age distributions of larvae, further supporting the hypothesized transport route.
Two major oceanographic regimes occupy the eastern seaboard of the United States. The southeast U.S. continental
shelf (SEUSCS) extends from the Straits of Florida to Cape
Hatteras and contains relatively warm, salty water (Atkinson
et al. 1983). The northeast U.S. continental shelf (NEUSCS)
reaches from Cape Hatteras to Georges Bank and contains
relatively cold, fresher water (Beardsley et al. 1985; Aikman
et al. 1988; Churchill et al. 1993). (The terminology southeast and northeast U.S. continental shelves is used in the
Large Marine Ecosystem literature [Sherman 1993] and is
used here to avoid problems associated with the unofficial
and inaccurate geographical names South and Middle Atlantic Bight [Richards 1999]. Both sets of place names refer to
nearly identical geographic areas.) On the inner portion of
the SEUSCS, circulation is predominantly wind and buoyancy driven (Blanton and Pietrafesa 1978; Lee et al. 1989;
Werner et al. 1999). On the outer portion of the SEUSCS,
circulation is dominated by the Gulf Stream, which flows
northward along the shelf edge; flow direction at a given
location, however, is variable owing to wavelike meanders
(Lee et al. 1989). Large-scale circulation on the NEUSCS is
not directly influenced by the Gulf Stream, which separates
from the shelf edge in the vicinity of Cape Hatteras (Csanady
and Hamilton 1988). The mean current over the NEUSCS
and upper slope is directed southwestward, along isobaths,
and is associated with an along-shore pressure gradient
(Beardsley et al. 1985; Aikman et al. 1988). The NEUSCS
and SEUSCS flows converge near Cape Hatteras, and most
shelf and slope water is entrained into Gulf Stream flows
and carried northeastward (Churchill and Berger 1998).
The water mass boundary near Cape Hatteras coincides
with a boundary separating two distinct marine faunas: a
warm-temperate fauna to the south and a cold-temperate fauna to the north (Briggs 1974). Members of these faunas,
however, move between oceanographic regimes. Adults of
some cold-temperate species are found south of Cape Hatteras (e.g., tautog, Tautoga onitis, Parker and Dixon 1998),
and larvae of NEUSCS-spawning species can be transported
south of Cape Hatteras via wind and buoyancy-driven intrusions into the SEUSCS (Stegmann and Yoder 1996; Grothues et al. 2002). Likewise, many warm-temperate species
are found in the NEUSCS during late spring, summer, and
early fall (Able and Fahay 1998). Some of these warm-temperate species migrate northward into the NEUSCS as adults
in spring and return south with the onset of fall (e.g., Atlantic
menhaden, Brevoortia tyrannus, Ahrenholz 1991). However,
many warm-temperate species are transported north of Cape
Hatteras as larvae or juveniles (Markle et al. 1980; Able and
Fahay 1998). Larvae of some SEUSCS-spawned species use
nursery habitats in the NEUSCS and migrate southward in
fall to join adult populations (e.g., bluefish, Pomatomus saltatrix, Kendall and Walford 1979), whereas others are ex-
patriates and perish as water temperatures cool (e.g., spotfin
butterflyfish, Chaetodon ocellatus, McBride and Able 1998).
The physical mechanisms that transport larvae from the
SEUSCS to NEUSCS are important to the ecology of many
warm-temperate species. For species whose larvae and juveniles successfully utilize NEUSCS habitats, the mechanisms responsible for northward transport could control larval supply to nursery habitats (Hare and Cowen 1996). For
species whose larvae are expatriated to the NEUSCS, northward larval transport represents a loss of propagules from
southern populations. Under a scenario of climatic warming,
expatriation may provide a mechanism for warm-temperate
species to colonize areas north of Cape Hatteras, similar to
the recent spread of many tropical species into the northern
SEUSCS (Parker and Dixon 1998). Larval transport from
the SEUSCS to the NEUSCS might also be important in the
northward dispersion of introduced species (Whitfield et al.
2002).
Several potential physical mechanisms could result in the
transport of SEUSCS and Gulf Stream water, and their
plankton, to the NEUSCS. Pietrafesa et al. (1994) found that
strong northward winds can force SEUSCS water northward
into the NEUSCS, in opposition of the NEUSCS’s mean
current. However, such wind-driven excursions have not
been observed more than 100 km north of Cape Hatteras
and cannot account for observations of SEUSCS-spawned
fish larvae in the central or northern NEUSCS (Shima 1989;
Hare and Cowen 1991; Cowen et al. 1993), or still further
to the northeast over Georges Bank (Cowen et al. 1993) and
the Scotian Shelf (Markle et al. 1980). Water discharged
from Gulf Stream meanders often makes its way to the
NEUSCS edge (Churchill and Cornillon 1991a, 1991b;
Churchill et al. 1993). Using hydrographic data acquired
quarterly over 11 yr, Churchill and Cornillon (1991b) determined that the probability of such transport decreases sharply going northwards from Cape Hatteras, and found no intrusions of discharged Gulf Stream water at the NEUSCS
edge north of 398N (off Delaware Bay).
Many investigators have postulated that warm-core rings
(WCRs), which are mesoscale eddies that form when Gulf
Stream meanders ‘‘pinch-off’’ (Joyce et al. 1984), transport
subtropical and warm-temperate plankton, including larvae,
to the northeast North American shelf edge (Cox and Wiebe
1979; Markle et al. 1980). Specifically, Hare and Cowen
(1991, 1996) and Cowen et al. (1993) proposed four segments for northward larval transport associated with WCRs
(Fig. 1). Larvae that are spawned on the continental shelf
south of Cape Hatteras are (1) entrained into the Gulf
Stream, (2) transported to the northeast along the edge of
the Gulf Stream, (3) carried in WCR streamers from the Gulf
Stream and across the Slope Sea (i.e., the region between
the Gulf Stream and the shelf edge north of Cape Hatteras),
and (4) ejected from WCR streamers at the shelf edge where
1776
Hare et al.
Fig. 1. Transport route proposed by Hare and Cowen (1991,
1996) and Cowen et al. (1993) for the movement of larvae spawned
along the southeast U.S. continental shelf (SEUSCS) to the northeast U.S. continental shelf (NEUSCS) edge. Larvae that are
spawned on the northern SEUSCS are (1) entrained into the Gulf
Stream, (2) transported to the northeast along the edge of the Gulf
Stream, (3) carried in WCR streamers from the Gulf Stream across
the slope region between the Gulf Stream and the shelf edge north
of Cape Hatteras, and (4) ejected from WCR streamers at the shelf
edge, where they enter the NEUSCS/slope frontal region. Coastline
and 200-m isobath are plotted, and place names used in the text are
indicated.
they enter the shelf/slope frontal region. WCR streamers are
composed of Gulf Stream–derived water that moves around
the western periphery of the WCRs and result from a WCR–
Gulf Stream interaction (Evans et al. 1985; Nof 1988). Ages
of larval fish derived from otolith increments suggest that
transport over the proposed route can occur in as little as 8
d (Hare and Cowen 1991). Although founded in a general
understanding of the physical oceanography of the region,
there is no direct physical evidence for the proposed transport route (see discussion by Epifanio and Garvine 2001);
nor is there independent verification of the speed over which
water and larvae can travel over the proposed route.
The purpose of the present study was to evaluate the transport route proposed by Hare and Cowen (1991, 1996) and
Cowen et al. (1993) through an examination of water properties and Lagrangian drifter tracks. These observations reveal transport along each of the four segments of the proposed transport route, as well as transport connections
joining the segments. To further assess the likelihood that
SEUSCS-spawned larvae observed on the NEUSCS were
carried along this route, a probability model was developed
that estimated the age distribution of larvae arriving at the
NEUSCS edge at the endpoint of the route. Model input
included all the available measurements of drifter speed
along the various segments of the proposed route, distances
for each segment extracted from satellite-derived sea surface
temperature (SST) images, and estimates of larval mortality
Fig. 2. (A) Tracks of 13 satellite-tracked drifters released in the
northern portion of the SEUSCS as part of three separate field programs (see Data sources). These drifter tracks were used to examine
segments 1 and 2 of the proposed route (Fig. 1, Table 1). (B) Tracks
of 11 drifters released in the slope region of the NEUSCS (see Data
sources). These drifter tracks were used to examine segments 3 and
4 of the proposed route (Fig. 1, Table 1). Coastline and 200-m
isobath are plotted with drifter tracks.
rate. Model results were then compared with measured age
distributions of SEUSCS-spawned larvae captured at the
NEUSCS edge to assess the validity of the proposed transport route.
Materials and methods
Data sources—Hydrographic data: Hydrographic measurements taken along a cross-slope transect east of Virginia
during May 1984 as part of the Mid-Atlantic Slope and Rise
(MASAR) project (Csanady and Hamilton 1988) provided a
view of SEUSCS water at the edge of the Gulf Stream, north
of Cape Hatteras. CTD (conductivity, temperature, depth)
casts and XBT (expendable bathythermograph) drops were
made, and observed temperature–salinity (TS) properties
were compared to the TS properties of the water masses
typically observed in the SEUSCS and southern NEUSCS
during springtime (Atkinson et al. 1983; Churchill et al.
1993). To gain a sense of the speed of entrained SEUSCS
Lagrangian views of larval transport
1777
Table 1. Summary of drifters used in this study including original project, transmitter number,
release date, water depth at release, drogue type and depth, general release location (see Fig. 1),
and transport route segments for which the drifter was included in analysis. Drogue type refers to
holey sock (HS), surface (S), and window shade (WS). Drogue depth refers to depth of the center
of the drogue.
Project
FRED
FRED
FRED
FRED
SABRE
SABRE
SABRE
SABRE
MMS-CHFS
MMS-CHFS
MMS-CHFS
MMS-CHFS
MMS-CHFS
SEFCAR‡
DWDS-EPA
DWDS-EPA
DWDS-EPA
DWDS-EPA
DWDS-EPA
DWDS-EPA
DWDS-EPA
DWDS-EPA
DWDS-EPA
DWDS-EPA
DWDS-EPA
Drogue
Water
depth
Transmitdepth at
type
ter no. Release date release (m) (m)
8772
8773
8775
8776
9806
17543
2600
4563
3537
3540
3774
3590
3593
23112
22734
12738
12742
12745
12750
12761
12762
12763
12768
12769
12771
23
22
12
12
1
1
17
17
5
26
30
8
12
27
3
4
6
21
13
3
8
4
8
20
3
May 87
May 87
May 87
May 87
Feb 94
Feb 94
Dec 94
Dec 94
May 92
Aug 92
Oct 93
Nov 92
May 93
Feb 96
May 91
May 90
Jul 90
Jul 90
Sep 90
Dec 90
Dec 90
Nov 90
Nov 90
Jan 91
Feb 91
130
270
30
35
30
30
30
30
25
25
25
25
25
.2000
.2000
.2000
.2000
.2000
.2000
.2000
.2000
.2000
.2000
.2000
10
10
10
10
0
0
19
9
2.25
2.25
2.25
2.25
2.25
2.25
10
10
10
10
10
10
10
10
10
10
10
HS
HS
HS
HS
S
S
WS
WS
HS
HS
HS
HS
HS
HS
HS
HS
HS
HS
HS
HS
HS
HS
HS
HS
HS
SegRelease location ment
Onslow Bay
Onslow Bay
Raleigh Bay
Raleigh Bay
Onslow Bay
Onslow Bay
Onslow Bay
Onslow Bay
Raleigh Bay
Raleigh Bay
Raleigh Bay
Raleigh Bay
Raleigh Bay
Florida Bay
106 DWDS
106 DWDS
106 DWDS
106 DWDS
106 DWDS
106 DWDS
106 DWDS
106 DWDS
106 DWDS
106 DWDS
106 DWDS
1,2
1,2
1,2
1,2*
1,2
1,2
1†
1,2
1,2
1,2
1,2
1,2
1,2
3,4
3§
3§
3,4
3,4
3,4
3§
3,4
3,4
3,4
3,4
* Crossed from the Gulf Stream to the northeast U.S. continental shelf edge in a discharge of water emanating
from a Gulf Stream meander.
† After release, drifter moved southwest and was stranded on Frying Pan Shoals. This drifter was omitted from
the residence time on the southeast U.S. continental shelf (segment 2).
‡ Released off south Florida, subsequently became entrained into Gulf Stream flows, and moved north of Cape
Hatteras, where it was incorporated into a WCR during ring formation.
§ One of three EPA-DWDS drifters that circuited around the periphery of a WCR and was not detrained in the
vicinity of the shelf/slope front north of Cape Hatteras.
water, the geostrophic velocity component normal to the
transect line was calculated using the CTD station at which
SEUSCS water was encountered and the adjacent XBT stations (the tight TS relation observed in the transect data allowed us to infer salinity from the XBT data).
Drifter data: Tracks of 13 drifters released in the northern
SEUSCS during three separate experiments (Fig. 2A, specifics of drifters and their release are provided in Table 1)
were used to examine movements along segments 1 and 2
of the proposed transport route (Fig. 1). Four of the tracks
were from the Frontal Eddy Dynamics (FRED) experiment,
which examined Gulf Stream frontal eddies between Cape
Fear and Cape Hatteras (Glenn and Ebbesmeyer 1994a,
1994b). Four tracks were from the South Atlantic Bight Recruitment Experiment (SABRE), which examined biological
and physical processes involved in the recruitment dynamics
of estuarine-dependent fishes that spawn on the SEUSCS
(Werner et al. 1999). The final five drifter tracks were ac-
quired as part of the Mineral Management Service–Cape
Hatteras Field Study (MMS-CHFS), which evaluated the potential effect of oil and gas exploration over the shelf region
between Chesapeake Bay and Cape Lookout (Churchill and
Berger 1998).
Drifter tracks from the Environmental Protection Agency’s (EPA) Monitoring, Research, and Surveillance Study of
the 106-mile Deep Water Dumpsite (EPA-DWDS) were used
to evaluate segments 3 and 4 of the proposed transport route
(Fig. 1). The EPA-DWDS evaluated the potential effects of
sewage sludge dumping at a site on the continental slope
east of New Jersey and south of New York (Berger et al.
1996; Dragos et al. 1996). During the study, 58 drifters were
deployed in and around the 106-mile site. Tracks of 11 drifters, which were entrained in the Gulf Stream and then into
a WCR west of 658W, were used in this study (Table 1, Fig.
2B).
Location data from all drifters were quality checked and
linearly interpolated to produce tracks at evenly spaced in-
1778
Hare et al.
Fig. 4. (A) Distribution of speeds as drifters crossed out of the
SEUSCS. (B) Distribution of residence times of drifters released on
the SEUSCS. All drifters released on the SEUSCS were entrained
into Gulf Stream flows and moved rapidly to the northeast. One
drifter that was stranded on Frying Pan Shoals for about 1 week
(Table 1) was not included in the determination of residence time.
tervals (6 h). Velocities were then estimated from the tracks
by dividing the distance traveled in an interval by time.
Fig. 3. Five-day composite satellite-derived SSTs from June,
July, and August 1988. Station locations are plotted where P. saltatrix pelagic juveniles were captured in June 1988 and X. novacula
larvae were captured in July and August 1988. Symbol size represents concentration of larvae—zero larvae are denoted with an open
circle. The proposed transport route is indicated by a black line,
and distances were extracted from these lines for use in a probability
model. Red indicates warmer water and blue indicates cooler water.
Land is shown as white, and the 200-m isobath is plotted.
Satellite-derived SSTs: Drifter tracks were superimposed
on concomitant satellite-derived images of SSTs to relate
surface temperature features with movement of drifters over
different segments of the proposed transport route. Satellitederived SST images were obtained from the Southeast
CoastWatch Program at the NOAA Center for Coastal Fisheries and Habitat Research, from the Remote Sensing Laboratory at the University of Rhode Island, and from a data
set of 5-d composite imagery developed for NOAA by
Brown and Podesta (1989).
Probability model—Conceptual framework: A probability
model was developed to give estimates of the age distribution of larvae that traveled along the proposed route to arrive
Lagrangian views of larval transport
at a specified location on the NEUSCS edge. Specific scenarios were developed for the model to correspond to specific times when SEUSCS-spawned larvae were collected on
the outer NEUSCS. Modeled age distributions were then
compared to observed age distributions, which were determined through otolith aging of captured larvae. If actual larval ages appeared at very low probabilities in the modeled
age distribution, then transport along the proposed route
could be deemed unlikely. Conversely, coincidence of actual
ages with modeled ages of high probability supports the assertion that larvae had traversed the proposed route.
Conceptually, the model consists of two components: (1)
probability distributions of transport times over the four segments of the proposed route and (2) a larval mortality function. To determine component 1, a transit time probability
function was defined for each route segment (Fig. 1). For
route segment i, this is denoted as p i(t), where p i(t)dt is the
probability that larvae are transported the length of the segment in the forward direction (leading to the NEUSCS) over
a period between t and t 1 dt. Consider transport across the
first two segments. If p1(t) and p2(t) are independent and the
probability of going from one segment to the next is 1, then
the likelihood per unit time that a larva may transit the two
segments in a period t is simply the sum of all possible
combinations of p1(t1) p2(t2)dt, with t1 and t2 summing to t.
This can be expressed as a convolution integral,
p1–2 (t) 5
E
t
p1 (t 2 «)p2 («) d«
(1)
0
where p1–2(t)dt is the probability of being carried in the forward direction over segments 1 and 2 within a period of t
to t 1 dt, and « is a dummy variable for integration. With
the inclusion of additional segments, the above is expanded
to nested convolution integrals. The transit time distribution
for the four segments of the specified route is
p1–4 (t)
EE E
c
t
5
0
0
1779
1). Larvae were assumed to be spawned in the northern portion of the SEUSCS, and leaving the shelf is the first segment of transport to the NEUSCS.
Transport time along the Gulf Stream’s edge, p2(t): For
this, and the subsequent route segments, transit time probability was determined from (1) the segment length and (2)
the distribution of speeds along the segment. Segment length
was determined for three specific scenarios (June, July, and
August 1988), times when SEUSCS-spawned fish were collected on the outer shelf of the NEUSCS. The procedure
involved tracing the proposed route over SST images that
coincided with larval collections (Fig. 3) and then determining the distances along the edge of the Gulf Stream (segment
2), from the Gulf Stream to the NEUSCS edge around a
WCR (segment 3), and along the NEUSCS edge from the
WCR to larval collection locations (segment 4).
The speed distribution for segment 2 was determined from
the tracks of drifters that moved from 75 to 718W (range
chosen to include maximum number of drifters). The along–
Gulf Stream speed was taken as the ratio of the length of
the line connecting the crossings of these longitudes for each
drifter and the time difference between the crossings. This
method was necessary to give a representative speed because
movement of drifters along the edge of the Gulf Stream often
was not unidirectional. Speeds were computed from FRED,
SABRE, and MMS-CHFS drifters.
The mean and standard deviation of the drifter speeds
were used to specify a normal transport speed distribution.
This was defined as q2(v), where q2(v)dv is the likelihood
that larvae traverse segment 2 with a mean speed between
v and v 1 dv. Because our interest was confined to transport
in the forward direction along the route, this distribution was
converted to one defined for speeds in the forward direction
only. To ensure that the ‘‘forward speed’’ distribution would
integrate to unity, it was determined as
q9(v)
5
2
«
p1 (t 2 c)p2 (c 2 «)p3 (« 2 k)p4 (k) dk d« dc
0
E
q2 (v)
`
(4)
q2 (v) dv
0
(2)
with k and c also dummy variables of integration.
A survival probability S(t) was defined to account for the
effect of mortality on the final transit time distribution (component 2). This gives the likelihood of survival for larvae
that cover the route in a time t. The transit time probability
function with mortality included then becomes Eq. 3.
P(t) 5 p1–4(t)S(t)
Conversion of q92 (v) to a transit time probability was done
by noting that the equation
q92(v)dv 5 p2(t)dt
(5)
is satisfied for v 5 D2 /t, where D2 is the segment 2 length.
Evaluating dv/dt gives Eq. 6.
p2(t) 5 D2q92(v)/t 2
(6)
(3)
Residence time on the SEUSCS, p1(t): This segment was
represented as a normal distribution of the expected residence time of water on the SEUSCS. This distribution was
fully specified by the mean and standard deviation of residence time of drifters deployed on the SEUSCS. Residence
time was defined as the period between a drifter’s release
and its crossing of 758W. FRED, SABRE, and MMS-CHFS
drifters were used, but the SABRE drifter that grounded on
Frying Pan Shoals was excluded from the calculation (Table
Warm-core ring transport time to the shelf edge, p3(t):
None of the 13 drifters released in the SEUSCS were incorporated into WCR streamers, and only two of these drifters passed a WCR. Thus, these drifters could not be used to
examine segments 3 and 4 of the proposed transport route
(Table 1).
Examination of drifter tracks with concomitant SST images indicated that at least 23 of the EPA-DWDS drifters
moved from the edge of the Gulf Stream into the circulation
of a WCR before reaching 608W. Analyses herein were con-
1780
Hare et al.
Fig. 5. (A) Location of hydrographic stations sampled during MASAR cruise 2 (Csanady and Hamilton 1988). (B) Section of temperature and (C) salinity along the MASAR transect extending from the southern portion of the outer NEUSCS to the edge of the Gulf Stream.
(D) Volumetric TS diagram of water found over the SEUSCS during May (from Atkinson et al. 1983). The number in each square gives
the volume (km3) of SEUSCS water with TS characteristics falling within the box’s range. Also shown is the TS profile of water encountered
at station M24. The near-surface portion of the M24 trace overlaps with data from the SEUSCS. (E) TS regimes of various water masses
found in the southern portion of the NEUSCS during late May and early June 1984 (from Churchill et al. 1993). TS profile from station
M24 is overlain. The near-surface portion of the M24 trace does not overlap with data from the NEUSCS.
ducted on only the 11 drifters that entered WCR circulation
west of 658W (Fig. 2B). WCR streamer speed was calculated
as the average of a drifter’s speed while in a WCR streamer.
The resulting ensemble of drifter speeds was used to specify
p3(t) in the manner as described above for p2(t).
Transport time along the NEUSCS edge, p4(t): Transport
speed along the NEUSCS edge were calculated from EPADWDS drifters that exited WCR streamer circulation and
moved to the west-southwest parallel to the NEUSCS edge
(n 5 8). For each drifter, along-shelf edge speed was calculated as its average speed while it traveled parallel to the
shelf edge. The speed ensemble was used to specify p4(t) in
the manner as described above for p2(t).
Mortality: Following Peterson and Wroblewski (1984),
larval fish survival probability was modeled as an exponential function. The decline of a population with an initial N0
individuals can be expressed as N(t) 5 N0e 2zt, where N(t) is
the number of individuals surviving to time t and z is the
instantaneous larval mortality rate. The survival probability
function was expressed relative to the overall survival likelihood (for all transit times) as
S(t) 5 e 2zt /[#`0 p1–N(t)e 2ztdt]
(7)
Scenario-specific mortalities were calculated from the
slope of the natural logarithm–transformed age distribution
(Ricker 1975). For Xyrichtys novacula larvae (the July and
August 1988 scenarios), mortality rate was estimated using
Lagrangian views of larval transport
1781
Fig. 6. (A) Tracks of two FRED drifters released in the northern SEUSCS. Track time is coincident with the underlying satellite SST
image. Circles indicate locations at 24-h intervals. (B) Speed time series derived from the two drifter tracks shown in panel A. Both drifters
moved east-northeastward in association with the Gulf Stream front at speeds between 100 and 200 cm s21. (C, E) Tracks of two MMSCHFS drifters released in Raleigh Bay. Both drifters exit the SEUSCS in association with the Gulf Stream; however, both are detrained
from the main Gulf Stream current downstream of Cape Hatteras in association with meander crests. Both were then re-entrained. Track
time is coincident with the underlying satellite SST images. Circles indicate locations at 24-h intervals. (D, F) Speed time series derived
from the two drifter tracks shown in panels C and E, respectively. Time at which drifter leaves SEUSCS is indicated by vertical line labeled
SEUSCS. Periods that correspond to positions and images shown in panels C and E are indicated by black bars.
data presented in Hare and Cowen (1991). Estimated instantaneous mortality was 0.12 6 0.01 d21 (mean 6 SE, r 2 5
0.89). For P. saltatrix, the age distributions of P. saltatrix
larvae collected in the SEUSCS during April 1989 and
March 1990 were used (Hare and Cowen 1996, unpubl.
data). Estimated P. saltatrix mortality was 0.11 6 0.02 d21
(r 2 5 0.67).
Analysis of model results: The probability model estimated larval age distributions for three scenarios in which
SEUSCS-spawned larvae were collected over the outer
NEUSCS. Shima (1989) reported the ages of SEUSCSspawned P. saltatrix pelagic juveniles that were collected in
the vicinity of the Hudson Canyon during June 1988. Hare
and Cowen (1991) reported the ages of expatriated X. novacula larvae that were captured in the Hudson Canyon region during July and August 1988 (only larvae collected at
the outer three stations of the cross-shelf sampling grid were
included in the comparison with model results).
Sensitivity analyses were conducted to evaluate the effect
of various model parameters on predicted larval age distributions. Larval mortality rate (z) was varied for each of the
three time periods modeled: June, July, and August 1988.
Values of z used in the sensitivity analyses were 0.05, 0.2,
0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 d21, which covers the
range of larval mortality rates commonly reported (Peterson
and Wroblewski 1984; Morse 1989). Distance of the route,
mean speed, and standard deviation of speed were also varied for each of the three time periods modeled. Values were
varied by 610, 25, 50, 75, and 100%. Values were varied
as an ensemble; for example, all distances were increased
by 10% or all speeds were decreased by 25%. The number
of observed ages that were in the lower and upper 5% tails
of each modeled distribution was then calculated.
Results
Entrainment of water from the SEUSCS into the Gulf
Stream—Upon release on the SEUSCS, most drifters moved
to the northeast (n 5 10); however, some initially moved to
1782
Hare et al.
Fig. 7. Distributions of Gulf Stream–associated speeds for drifters released on the SEUSCS.
the southwest before turning to the northeast (n 5 3) (Fig.
2A). All drifters eventually exited the SEUSCS into the vicinity of the western Gulf Stream front and exit speeds were
typical of reported Gulf Stream and Gulf Stream frontal velocities (;100 cm s21; Fig. 4A; Glenn and Ebbesmeyer
1994a, 1994b). No particular SST feature could be linked to
the process of Gulf Stream entrainment owing to cloud cover
during entrainment events. Defining 758W as the eastern extent of the SEUSCS, the mean drifter residence time on the
outer SEUSCS was 8.5 d (Fig. 4B).
SEUSCS water at the Gulf Stream’s edge north of Cape
Hatteras—The temperature distribution along the MASAR
transect (Fig. 5A) showed the Gulf Stream front approximately 130 km east of the shelf edge (Fig. 5B). A surface
layer of relatively fresh water extending to a depth of about
10 m was situated along the western edge of the Gulf Stream
front (station M24; Fig. 5C). TS characteristics identified this
layer as entrained SEUSCS water and were within the range
of TS properties of water found on the SEUSCS during May
(Fig. 5D) but were far outside the range of TS properties of
NEUSCS and slope water during May–June (Fig. 5E).
Estimates of geostrophic velocity normal to the transect
indicated that this entrained SEUSCS water was traveling at
a speed of roughly 200 cm s21. Between stations M23 and
M24, estimated geostrophic velocity at 5 m was 195 cm s21
(750 m reference depth); between stations M24 and M25,
estimated geostrophic velocity at 5 m was 211 cm s21 (330
m reference depth).
Drifter tracks supported the geostrophic estimate of
SEUSCS water speed along the edge of the Gulf Stream but
indicated that the path of this water can be complex. Most
drifters entrained from the SEUSCS into the Gulf Stream
remained in the Gulf Stream frontal region after passing
Cape Hatteras (Fig. 6A). The speed of all drifters exceeded
100 cm s21 at some point downstream of 758W (Fig. 6B),
and several traveled in excess of 200 cm s21. However, few
of the drifters maintained rapid downstream transport for
extended periods of time; many drifters either slowed in the
Gulf Stream or left and then reentered the Gulf Stream current (Fig. 6C–F). Detrainment of drifters from the main Gulf
Stream current occurred to both sides of the Gulf Stream
(e.g., into the slope region between the Gulf Stream and the
NEUSCS edge, and into the Sargasso Sea) and typically occurred near the crest of a meander. Similar detrainment and
entrainment of deeper RAFOS drifters has been described
by Bower and Rossby (1989). As a result of the detrainment
from and subsequent re-entrainment into the main Gulf
Stream current, the average speed between 75 and 718W was
101.1 cm s21 (Fig. 7).
Cross-slope movement of water from the Gulf Stream edge
to the NEUSCS edge—Evidence of WCR-associated transport of water from the edge of the Gulf Stream to the shelf
edge of the NEUSCS is frequently seen in satellite-derived
SST images (Joyce et al. 1984; Evans et al. 1985). Images
from June to August 1988 provide relevant examples of
WCR streamers (Fig. 3), covering the period during which
P. saltatrix and X. novacula larvae were captured near Hudson Canyon (Shima 1989; Hare and Cowen 1991, 1996).
These SST images show a series of rings moving southwestward through the NEUSCS slope region, each with a
band of entrained Gulf Stream water.
Drifter tracks from the EPA-DWDS study show evidence
of cross-slope transport in WCR streamers. Upon release
over the NEUSCS upper slope, most of the 58 drifters
moved to the southwest. All were eventually entrained into
the Gulf Stream. Once in the Gulf Stream, these drifters
moved northeastward at the edge of the Gulf Stream, similar
to the drifters released in the SEUSCS (Fig. 2B). Eleven of
the EPA-DWDS drifters were incorporated into WCR
streamers west of 658W, some directly from the Gulf Stream
→
Fig. 8. (A–C) Track of an EPA-DWDS drifter released off the shelf of the NEUSCS. The drifter initially moved to the southwest before
becoming incorporated into the flows associated with the Gulf Stream in the vicinity of Cape Hatteras (not shown). The drifter traveled
east-northeastward with the Gulf Stream but was then detrained in association with a Gulf Stream meander. Within 4 d, the drifter was
entrained into the cross-slope circulation of a WCR streamer. After crossing the Slope Sea, the drifter exited the WCR streamer and moved
westward parallel to the NEUSCS edge. In panels A, B, and C, red indicates warmer water and blue indicates cooler water. The black line
shows the entire track. Black circles show daily positions concomitant with the 5-d composite image. (D) Speed time series derived from
the drifter track shown in panels A–C. The period corresponding to panels A–C are indicated by black bars. (E) Distribution of cross-slope
speed associated with the WCR streamer for the ensemble of 11 EPA-DWDS drifters. (F) The track of a drifter released off of South
Florida in January 1998. Once north of Cape Hatteras, the drifter moved northeastward with the Gulf Stream. The drifter was then
Lagrangian views of larval transport
1783
incorporated into the interior of a WCR upon formation, where it remained for 30 d. Note the difference in the fate of this drifter relative
to those that are transported across the slope in association with WCR streamers (Fig. 8A–C). Temporal series of track times are coincident
with the underlying satellite SST images. Circles indicate locations at 24-h intervals. Red indicates warmer water and blue indicates cooler
water. (G) Track of a FRED drifter released in the northern SEUSCS in May 1987. The drifter moved across the slope region to the
NEUSCS edge in association with a Gulf Stream discharge. The black line shows the entire track. Black circles show daily positions
concomitant with the 5-d composite image. (H) Speed time series of FRED drifter shown in panel G. Period corresponding to panel G is
indicated by a black bar.
1784
Hare et al.
H). This observation indicates that although the probability
of cross-slope transport in a Gulf Stream discharge decreases
from Cape Hatteras northward (Churchill and Cornillon
1991b), such transport can occur north of Delaware Bay.
Expulsion of water from a WCR at the NEUSCS edge—
Drifter tracks indicate that the probability of water in a WCR
streamer being ejected from the ring into the shelf edge region is high. Eight of the 11 EPA-DWDS drifters left the
WCR circulation near the NEUSCS edge and moved westsouthwestward (Fig. 8C). The remaining three drifters continued around the WCR and were eventually re-entrained
into the Gulf Stream current. Once along the NEUSCS edge,
all eight drifters initially moved to the west-southwest with
an average speed of 15.4 cm s21 (Fig. 9).
Fig. 9. Distribution of along-NEUSCS speeds for EPA-DWDS
drifters that crossed the slope region in association with WCR
streamers and were ejected from WCR circulation in the vicinity of
the NEUSCS edge.
front and others after detrainment from the Gulf Stream in
association with a Gulf Stream meander (Fig. 8A,B). Following entrainment into a WCR streamer, transport across
the Slope Sea was direct, with an average speed of 50.9 cm
s21 (Fig. 8D,E).
Cross-slope transport of water in WCR streamers was distinct from the circulation of the ring itself. An example was
provided by a drifter released off of the Florida Keys that
traveled along the northern edge of the Gulf Stream during
April 1998 (Fig. 8F). This drifter moved into a meander crest
as a WCR formed and remained in the anticyclonic circulation of the ring for at least 30 d. Such transport would trap
larval fish in the ring until either the ring decayed or larvae
developed the ability to actively move out of the ring.
One drifter was ejected from the Gulf Stream near the
crest of a meander and moved across the Slope Sea to the
NEUSCS edge within a filament of discharged Gulf Stream
water (Fig. 8G). While in the filament, the drifter’s crossslope speed was slower (;25 cm s21) than typical crossslope speeds within WCR streamers (;50 cm s21) (Fig. 8D,
Table 2. Summary of specific transport distances used for each
transport segment for parameterization of the probability model.
Transport distances were determined by tracing the proposed transport route (Fig. 1) on satellite-derived SST images and extracting
distances (Fig. 3). Mortality values used in the model are also provided. Mortality rates were determined from abundance-at-age data
(Ricker 1975).
Transport model
segment
Segment
No.
Gulf Stream
Cross-slope
Along-NEUSCS
2
3
4
Mortality rate (z)
Transport distance (km)
Jun 88
Jul 88
Aug 88
668
280
172
553
313
0
455
285
0
0.11
0.12
0.12
Probability model—The probability model used distances
determined from SST imagery (Fig. 3, Table 2), mortality
rates calculated from abundance at age (Table 2), and speeds
determined from drifter statistics (Table 3). One potential
concern with the model was the small number of observations used to determine the speed components. However, the
values reported here agree with observations from a number
of studies that used a variety of techniques (Table 4). Thus,
we have confidence in the values used here.
Distributions of observed larval ages closely matched larval age distributions derived from the probability model
(Fig. 10). Means of observed larval ages were within 4 d of
mean modeled ages, and standard deviations of observed and
modeled ages were within 3.5 d (Fig. 10). Patterns in observed larval age distributions between sampling times were
also replicated in the modeled transit time distributions.
Maximum modeled and observed ages were of P. saltatrix
caught in June 1988 (Fig. 10). The older modeled ages of
this scenario resulted from the length of the along-NEUSCS
component (Fig. 3, Table 2). Minimum modeled and measured ages were for X. novacula caught in August 1988 (Fig.
10). The young modeled ages resulted from shorter Gulf
Stream and cross-shelf distances and from the absence of an
along-NEUSCS segment (Fig. 3, Table 2).
Larval mortality was an important component of the model (Fig. 11). At the lowest mortality (z 5 0.05), the means
and standard deviations of the modeled ages exceeded the
means and standard deviations of observed larval ages (Fig.
11, top row). As larval mortality increased, the mean and
standard deviation of model ages decreased. Model results
were generally consistent with observations over a range of
larval mortality from 0.1 to 0.3 (Fig. 11), which agrees with
the mortality rates estimated in this study (Table 2) and with
mortality rates reported in the literature (Peterson and Wroblewski 1984; Morse 1989).
Sensitivity analyses also demonstrated that the best fit between modeled and observed larval ages occurred at model
distance and speed values within 25% of calculated values
(Tables 2, 3). Standard deviations in speed had little effect
on modeled distributions (Fig. 11, second row). For transport
distances and mean speeds, changes of 650, 75, and 100%
resulted in modeled larval ages that were different than observed larval ages (Fig. 11, third and fourth rows); a large
number of observed ages fell within the lower and upper 5%
Lagrangian views of larval transport
1785
Table 3. Summary of general transport speed values used for each transport segment for parameterization of the probability model. Transport speed values were derived from drifter tracks analyzed in combination with SST images. Segment numbers refer to Fig. 1. SEUSCS residence time
is the residence time of a drifter on the SEUSCS. Along-NEUSCS speed is the speed along the
NEUSCS shelf/slope frontal region.
Transport model segment
Segment
Number
SEUSCS residence time
Gulf Stream speed
Cross-slope speed
Along-NEUSCS speed
1
2
3
4
Mean
8.5
101.1
50.9
15.4
tails of the modeled distributions. Greater transport speeds
and lesser transport distances had less of an effect on the
modeled age distribution of X. novacula larvae in July and
August 1988, likely because larvae cannot be transported to
the NEUSCS edge much faster.
One potential concern with the model is that the transport
distances are for one point in time; yet, as larvae move over
the route, transport distances change. If we accept the distances from June 1988 (July 1988) to represent the maximum change that could have occurred in transport distances
for larvae that were collected in July 1988 (August 1988),
the sensitivity analysis indicates that modeled ages would
not be greatly affected by changing route length because
distance varied by less than 25% between months (Table 2;
Fig. 11, bottom row).
Discussion
Drifter tracks and hydrographic data provide direct physical evidence of the proposed route by which larvae spawned
in the northern SEUSCS are entrained into the Gulf Stream,
transported to the northeast, regularly incorporated in WCRstreamers, transported across the slope region and released
along the NEUSCS edge. All drifters released in the northern
SEUSCS were entrained into Gulf Stream flows and were
carried rapidly to the northeast along the edge of the Gulf
Stream. Hydrographic data confirmed the presence and speed
of SEUSCS water along the edge of the Gulf Stream north
d
cm s21
cm s21
cm s21
SD
Sample size
6.4
49.9
13.5
4.9
12
13
11
8
Source
Fig.
Fig.
Fig.
Fig.
4
7
8
9
of Cape Hatteras. Similar observations of SEUSCS water
along the western Gulf Stream front north of Cape Hatteras
have been reported recently (Grothues and Cowen 1999;
Hare et al. 2001). Drifters were incorporated in WCR
streamers from the edge of the Gulf Stream and then released
from ring circulation in the vicinity of the NEUSCS edge.
All three observed larval age distributions fell within
modeled larval age distributions, which were based on drifter-derived transport speed distributions and SST image–derived transport distances. Sensitivity analyses found that
small changes in model parameters changed the modeled
distributions to more closely match observed distributions.
The coincidence of actual and modeled ages supports the
assertion that the larvae collected on the NEUSCS edge traversed the proposed route, and that the statistics of transport
speed over the route’s segments, derived from multiple years
of drifter track data, are relatively stationary (not changing
widely over time).
The probability of SEUSCS-spawned larvae moving over
the proposed route is largely dependent on WCR–Gulf
Stream interactions. All drifters released in the northern
SEUSCS were entrained into Gulf Stream flows. Approximately 75% of drifters moving in WCR streamers were released from streamers along the NEUSCS edge and traveled
southwestward along the shelf edge. Entrainment into a
WCR streamer is the most variable step. Although none of
the drifters released in the northern SEUSCS were incorporated into WCR streamers, only two passed WCRs. One
Table 4. Speed estimates for the various components of the proposed transport route as reported in other studies. The agreement between
velocities derived in this study (Table 3) and those of other studies provide confidence in the speed estimates used in the probability model.
Additionally, Nof (1988) indicated that propogation rate around a WCR in a streamer equals one-half of the particle speed along the
upstream front of the Gulf Stream. Based on the Gulf Stream speeds shown in this table, cross-slope velocities in WCR streamers would
be 20–75 cm s21, which agrees with the speed derived from drifter tracks (Fig. 8E). ADCP is Acoustic Doppler Current Profiler.
Transport model
segment
Gulf Stream
Cross-slope
Along-NEUSCS
Segment No.
2
2
2
2
3
3
4
4
4
Estimated speed
(cm s21)
40–150
100–130
40–130
30–150
51.1 (SD
40–80
16.4 (SD
10.2 (SD
12.4 (SD
5 26.7)
5 7.8)
5 8.1)
5 10.6)
Method (depth of
measurement, m)
PEGASUS (0)
ADCP (50)
ADCP (60)
ADCP (100)
SST (0)
ADCP (50)
Drifters
Current meter (10)
Current meter (10)
Source
Halkin and Rossby 1985
Hummon and Rossby 1998
Pierce and Joyce 1988
Joyce et al. 1990
Zheng et al. 1984
Hummon and Rossby 1998
Lozier and Gawarkiewicz 2001
Aikman et al. 1988
Beardsley et al. 1985
1786
Hare et al.
Fig. 10. The results of a probability model (line) compared to
observed larval ages (histogram). Modeled and observed mean and
standard deviation are provided. In all cases, the model results are
very similar to observed ages, supporting the hypothesis that the
larvae were transported from the SEUSCS to the NEUSCS edge
via the transport route proposed by Hare and Cowen (1991, 1996)
and Cowen et al. (1993) (Fig. 1).
interpretation is that the proposed transport route does not
occur. An alternative interpretation is that incorporation into
WCR streamers occurs at some probability ,1, and is fundamentally dependent on the presence of a WCR. This latter
interpretation is supported by the tracks of the EPA-DWDS
drifters. These drifters moved northeastward in the Gulf
Stream similar to the drifters released in the northern
SEUSCS. During the period of the EPA-DWDS study,
WCRs were prevalent (Berger et al. 1996) and 11 of 58
drifters were incorporated into WCR streamers west of
658W, indicating an approximate 20% probability of incorporation into a WCR. These data indicate that entrainment
from the Gulf Stream into WCRs depends, first, on the presence of a WCR and, second, on the location of a water parcel
(or drifter) relative to the spatial structure of the WCR–Gulf
Stream interaction (Hummon and Rossby 1998).
Because entrainment into a WCR streamer is the most
variable component of the proposed transport route, the supply of larvae from the SEUSCS to the NEUSCS edge is
largely controlled by entrainment into a WCR streamer. This
conclusion is consistent with the strong correlation between
WCR streamer activity and recruitment of SEUSCSspawned P. saltatrix to NEUSCS estuarine nursery habitats
(Hare and Cowen 1996), and the correspondence between
the location of WCRs and the occurrence of SEUSCSspawned larvae along the NEUSCS edge north of Delaware
Bay (Hare and Cowen 1991; Cowen et al. 1993). Larval
supply of a wide variety of warm-temperate species to juvenile habitats north of Cape Hatteras might be controlled
by the cross-slope transport in WCR streamers (Atlantic
menhaden, Brevoortia tyrannus; American eel, Angullia rostrata; conger eel, Conger oceanicus; crevalle jack, Caranx
hippos; C. ocellatus; white mullet, Mugil cephalus; striped
mullet, Mugil curema; butterfish, Peprilus triacanthus). Because WCRs are a universal element of western boundary
current systems (Olson 2001), the described transport route
could be an important source of larval supply to more poleward continental shelves throughout the world’s oceans.
Discharges of water from Gulf Stream meanders also appear to be an important mechanism of larval movement from
the Gulf Stream into the slope region of the NEUSCS (Hare
et al. 2001), but the likelihood of transport to the NEUSCS
edge via this mechanism decreases northward from Cape
Hatteras (Churchill and Cornillon 1991b). Discharges of
Gulf Stream water might explain the recruitment of
SEUSCS-spawned bluefish to NEUSCS estuaries in years of
low WCR streamer activity (Hare and Cowen 1996). One
drifter in this study provided an example of cross-slope
transport in discharged Gulf Stream water. If slower crossslope transport were indicative of all discharges of Gulf
Stream water, then fish arriving at the NEUSCS edge via
Gulf Stream water discharges would be older (slower speed)
and fewer in number (more cumulative mortality) than those
carried across the slope in WCR streamers.
The short resident times of drifters released in the northern
SEUSCS indicate that a majority of larvae spawned in this
region might be lost to the Gulf Stream. Drift bottle studies
(Barans and Roumillat 1976) and modeling studies (Checkley et al. 1999; Hare et al. 1999) indicate that the residence
time of surface water in the northern SEUSCS is short and
Lagrangian views of larval transport
1787
Fig. 11. The sensitivity of model results to variations in larval mortality (z) (first row), standard deviation of transport speed (second
row), mean transport speed (third row), and transport distance (fourth row). Columns are the results for the three different time periods
modeled: June 1988, July 1988, and August 1988. The observed mean is indicated by a horizontal dotted line for comparison to the model
results. Values used in the model (Fig. 10) are indicated by vertical dotted lines. The model mean (6SD) from the sensitivity analyses are
shown by black circles. The number of observed ages falling within the 5% upper and lower tails of the modeled distributions are shown
as the histograms.
decreases with distance from shore. Blanton and Pietrafesa
(1978) estimated a complete flushing time of Onslow Bay
of 60 d, with bottom waters moving in and surface waters
leaving. Govoni and Spach (1999) and Grothues and Cowen
(1999) estimated large fluxes of larvae off the SEUSCS. Future work should attempt to quantify loss relative to recruitment of species that spawn on the SEUSCS and depend on
SEUSCS nursery habitats.
Drifter tracks also support the idea that the transport of
larvae onto the NEUSCS at the surface is rare. Gawarkiewicz et al. (1996) noted high-salinity slope water above the
pycnocline on the outer shelf (between 80- and 100-m iso-
baths) and identified the potential forcing mechanisms as
wind and WCRs. No drifters examined here and only a few
of the total EPA-DWDS drifter set crossed the 100-m isobath
(Berger et al. 1996; Dragos et al. 1996). Yet, many fish larvae cross from the slope onto the NEUSCS, which indicates
that the physical mechanisms of cross-frontal exchange predominantly operate below 10 to 15 m. Cowen et al. (1993)
proposed that on-shelf movement of some species occurred
via an ontogenetic deepening of larval vertical distributions
coupled with slopewater intrusions along the summertime
NEUSCS thermocline (Flagg et al. 1994). For species that
cross onto the NEUSCS at the surface (e.g., bluefish, P. sal-
1788
Hare et al.
tatrix; white hake, Urophycis tenuis), Hare and Cowen
(1996) and Hare et al. (2001) proposed active horizontal
swimming as the mechanism. Drifter tracks indirectly support both mechanisms of cross-shelf transport by demonstrating that passive, surface slopewater to NEUSCS transport is rare, but does occur (see Gawarkiewicz et al. 1996).
Larval mortality played an important role in the predicted
transport times, influencing both the mean and standard deviation of predicted ages. Generally, larval mortality values
from 0.1 to 0.3 resulted in predicted transport times (and
standard deviations) that most closely matched observed
ages, and these values are similar to observed larval mortality rates (Morse 1989). Inclusion of larval mortality is
clearly important in estimating connectivity between local
populations (sensu Cowen et al. 2000).
In a recent review, Epifanio and Garvine (2001) discussed
the importance of local wind and buoyancy forcing in the
transport of larval fish and invertebrates along the eastern
coast of the United States. This present study and others
(Hare and Cowen 1991, 1996; Cowen et al. 1993; Lee et al.
1994; Stegmann and Yoder 1996; Govoni and Spach 1999)
indicate that processes related to the region’s western boundary current are also involved in larval transport, particularly
on the outer shelf and where the shelf is narrow. A difficulty
facing the advancement of larval transport studies in this
region, however, is a coupled understanding of the role of
locally and remotely forced processes in affecting larval
transport.
The approach developed here allows hypotheses regarding
larval fish transport to be tested. If observed larval ages
appeared at very low probabilities in the modeled age distribution, then transport along the proposed route could be
deemed unlikely. Although there are valid criticisms of using
Lagrangian measurements to follow larval fish (e.g., slippage
of drifter owing to wind, drifter tracking fixed depth versus
larval fish moving vertically), the large-scale view of larval
transport, both subjective and objective, provides valuable
insights into larval transport processes. A number of studies
have used Lagrangian measurements, both actual and modeled, to examine the mechanisms and outcomes of larval
transport (e.g., Pepin and Helbig 1997; Hare et al. 1999;
Cowen et al. 2000). Similarly, probabilistic approaches have
provided insights into the population-level consequences of
larval transport (Levin 1983; Schultz and Cowen 1994). As
these techniques continue to develop and are coupled with
field observations of larval fish distribution and abundance,
the ability to test hypotheses regarding larval transport
mechanisms will increase, thereby furthering our understanding of the importance of larval transport in the ecology
of marine populations.
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Received: 15 June 2001
Accepted: 1 May 2002
Amended: 4 June 2002